High purity silicon is needed for many kinds of electronic components such as silicon transistors, silicon integrated circuits and silicon solar cells. Since the invention of the first silicon transistor, many processes have been developed for producing silicon having the required purity levels.
A process that has been used for producing high quality metals such as steels, nickel-based superalloys, titanium etc. is known as a consumable electrode vacuum arc remelt (CEVAR) process. See, for example, U.S. Pat. No. 3,187,079 (Pestel); U.S. Pat. No. 3,344,840 (Buehl et al.); U.S. Pat. No. 3,480,716 (Lynch et al.); U.S. Pat. No. 4,303,797 (Roberts); U.S. Pat. No. 4,569,056 (Veil, Jr.); and United States Patent Application Publication No. 2008/0142188 A1 (Ishigami) for various technical aspects of the CEVAR process, all of which publications are incorporated herein in their entireties by reference. The CEVAR process is differentiated from a non-consumable electrode vacuum arc remelt where a non-consumable electrode, for example a graphite or tungsten electrode, is used to melt titanium or zirconium, for example, as disclosed in U.S. Pat. No. 3,546,348 (DeCorso). United States Patent Application Publication No. 2010/0154475 A1 (Matheson et al.) discloses a primary silicon purification process with similarities to the Kroll purification process of titanium with brief mention of a secondary silicon composition purification process that involves high temperature vacuum melting of a silicon composition that comprises a boron and phosphorus doped silicon with silicon purity in the range of 99.99 percent to 99.9999 percent.
Generally the CEVAR process produces a purified metal by these four steps: (1) evaporating impurities as the metal electrode is melted and exposed to a vacuum in the CEVAR furnace; (2) floating out of the liquid (melted) metal impurities that have a lower density than the metal electrode being melted; (3) dissociating molecular impurities by exposing them to the high energy plasma in the arc zone between the lower end of the electrode and the pool of molten (liquid) metal above the ingot being formed; and (4) solidification segregation, which results in impurity levels in the solidified metal of the ingot being lower for certain elements than the impurity levels in the adjacent liquid metal from which the solid ingot is being formed.
In the usual CEVAR process a room temperature metal electrode is charged into the CEVAR furnace, which is then evacuated to a vacuum. A high magnitude direct current (DC current) arc is then struck between the lower end of the electrode and the CEVAR water-cooled crucible. The arc causes the lower end of the electrode to melt, whereupon the molten metal falls into the closed bottom crucible, where it solidifies and then cools, to form a purified ingot.
Despite the ability of the CEVAR process to purify various metals, the process is not known to be used to purify a metalloid such as silicon. Since silicon is a semiconductor and not a metal in its relatively pure state (though in need of further purification for the above-mentioned end uses), it has a relatively high electrical resistivity at or near room temperature. In fact, a silicon electrode sufficiently pure to be a candidate for purification by the CEVAR process, would have an electrical resistance that is far too high to permit the passage of such a high arc current at any reasonable applied voltage when it is at or near room temperature.
The metal of the solidified ingot formed in the conventional CEVAR process is initially at its solidus temperature and then cools progressively within the water cooled crucible, with the edges of the ingot cooling more rapidly than the center due to the proximity of the edges to the adjacent water cooled wall of the crucible. This generates stresses in the ingot due to differential thermal contraction, a process that puts the ingot surface in tension and the center in compression. For the metals usually melted by the CEVAR process this is not a problem, since they are relatively ductile, that is, resistant to cracking. However, in the case of any conventional CEVAR process that is used to melt silicon, which is brittle over a wide range of temperatures, such an ingot would be prone to undesirable cracking.
It is one object of the present invention to provide apparatus and method for purification of a metalloid such as silicon that includes a CEVAR furnace and process.